Polyethylene terephthalate (hereinafter referred to as “PET”) is a resin well balanced in transparency, mechanical performance, solvent resistance, scent retaining properties, weather resistance, recyclability, and the like. Thus, PET is used for bottles, films, and the like. However, PET has the following drawbacks on heat resistance. More specifically, PET has a glass transition temperature of about 80° C. and hence cannot be used for the purpose where high heat resistance is demanded such as containers required sterilization or disinfection, heat resistant transparent cups for beverage, containers required reheating, or packaging materials for export products that are to cross the equator. Also, for UV barrier properties, PET transmits light at a wavelength of about 350 nm and thus the resin itself substantially has no UV barrier properties.
For this reason, polyethylene naphthalate (hereinafter referred to as “PEN”) has been often used in the fields where heat resistance is required.
Further, for the fields where UV barrier properties are required, PEN has been similarly used and the addition of a UV absorber to PET has been studied.
However, PEN is an expensive resin and hence has limited purpose of use. The blending of PET and PEN has been studied as a counter measurement but the refractive index (nd) of PET and PEN is 1.575 and 1.646, respectively, causing the refractive index difference to be as large as 0.071, whereby it was difficult to obtain a composition having excellent transparency from PET and PEN. Various methods for obtaining a transparent composition from PET and PEN have been studied, and proposals include a method for melt-kneading these resins in advance under predetermined conditions and the use of an extruder with extremely long residence time and good kneadability (e.g., see Patent Literature 1).
Further, Patent Literature 2 discloses a polyester resin modified with 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane and having excellent mechanical performance and moldability. Furthermore, a polyester resin composition containing the above polyester resin and other polyester resins and having excellent transparency, low crystallinity, melt viscoelasticity, heat resistance, and fabricability is proposed (e.g., see Patent Literatures 3 and 4).